Body Mass Index (BMI) is one of the most widely used health numbers — and one of the most misunderstood. It's a quick screening tool, useful at a population level, but it has real limitations for individuals. Here's what it measures, and what it doesn't.
How BMI is calculated
BMI is your weight divided by the square of your height. In metric units: weight (kg) ÷ height (m)². So 70 kg at 1.75 m is 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, multiply pounds by 703 and divide by height in inches squared — the 703 factor just converts to the same result.
BMI categories for adults
The World Health Organization uses these ranges for adults aged 20 and over. They're the same regardless of sex.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI doesn't tell you
BMI only uses height and weight, so it can't see what your weight is made of or where it sits. That leads to well-known blind spots:
- Muscle vs fat: muscle is denser than fat, so athletes and very muscular people can register as "overweight" despite low body fat.
- Fat distribution: BMI says nothing about where fat is stored, yet abdominal fat carries more health risk than fat elsewhere.
- Age and sex: body composition changes with age and differs by sex, which a single ratio can't capture.
- Population differences: health risks at a given BMI can vary across ethnic groups.
How to use BMI sensibly
Treat BMI as one rough signal among many — a starting point for a conversation, not a verdict. Trends over time, waist measurement, fitness, blood markers, and how you feel all add context BMI lacks. If your BMI flags a category you're unsure about, that's a reason to ask a professional, not to self-diagnose.